Seismic-refraction profiles in the Middle America Trench show that the main crustal layer ranges from normal oceanic thickness (about 5 km) t o half again as thick. Sediments are thicker than in the Pacific Basin. Because of the increased depth of water and sediment at all stations and thickening of the main crustal layer at some stations, the M discontinuity bows down beneath the trench. At the outer edge of the continental shelf near Guatemala, the water is replaced by sediments and rocks which may be either consolidated sediments or volcanic rocks; the M discontinuity is at little greater depth than beneath the trench.

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